1. Field of the Invention
The present invention relates to flywheels, and more particularly, to composite flywheels made from a unitary tape of liquid crystalline polymer.
2. Description of the Prior Art
Energy storage flywheels are being examined increasingly for use in transportation applications by making use of regenerative braking. Other uses for such flywheels are utility load leveling, storage of energy generated by solar or wind generators, and other similar uses.
The first flywheels used in such applications were made of metal. However, failure of metallic flywheels at high speeds often results in chunks of metal being released at high velocities. This necessitated the use of heavy safety housings to shield against such failures.
As an alternative to metallic elements, composite flywheels using ultrahigh strength fibers have been developed over the last few years. Such composites enable the power energy densities to be improved significantly while also allowing the use of lighter safety housings. The limitations of this approach are the slow, complex winding process; the need for post fabrication cure; and the packing density of the fibers which is limited in practice to 70 to 80 percent of the total volume. Moreover, composite flywheels, while possessing a more benign failure mode than metallic elements, still are subject to delamination at rotation speeds below which failure would occur due to tensile fracture under centrifugal loading.
Obviously, the faster a flywheel can be rotated, the more energy it can store. Thus it is advantageous to be able to produce composite flywheels which can be rotated as rapidly as possible. However, delamination can sometimes occur because the stress on an element varies as the square of the distance from the center of rotation. Assuming that the modulus of elongation of the elements making up the flywheel is constant throughout the flywheel, the strain in any individual element will be proportional to the square of the radial distance, since the types of fibers used are nearly linearly elastic (glass, graphite, and Kevlar (trademark of DuPont de Nemours and Company for an aromatic polyamide fiber)). Thus the elongation of elements at the periphery of the flywheel will be greater than that of elements closer to the axis. As a result, the flywheel breaks into concentric rings at rotation speeds below that which would be needed to break the individual elements. Accordingly, various attempts have been made to overcome this delamination problem.
For example, Rabenhorst, U.S. Pat. No. 3,964,341, describes a flywheel structure which is provided with a rim portion of multiple rings or winding of filamentary material having high tensile strength which are bonded or otherwise held together at a predetermined number of localized areas, so that major portions of the individual rings or windings may expand with a minimum of constraints during rotation of the flywheel structure.
Another approach is Rabenhorst, U.S. Pat. No. 3,982,447, which describes a flywheel formed of discrete annular rings of anisotropic filamentary material disposed within the rings in a "wound" configuration, wherein the upper and lower edges of each ring are attached alternately to the edges on adjacent rings in a radial orientation to form a structure having a bellows-like or convoluted cross-section. Such a structure is stated to be subject to unusually low radial stress within the material when rotating.
Rabenhorst, U.S. Pat. No. 4,020,714, describes a flywheel of wound filaments or discrete rings of essentially anisotropic material formed into a rim or disc-like configuration. Pairs of adjacent rings or windings in any given plane perpendicular to the axis of rotation are tied or bound together at discrete points at selected locations while adjacent rings which are not bound at these tie points are tied together at other points in between the first tie points. These points of connection extend linearly throughout the structure. Such an arrangement is stated to allow the individual rings to radially expand by bending in a radial direction thereby reducing the loading along the line of tie points as compared to a structure where all the rings or windings are tied together at all locations.
Rabenhorst, U.S. Pat. No. 4,023,437, discloses flywheels formed of wound anisotropic material. A disc-like flywheel is formed of windings of anisotropic filaments wherein the filaments are provided with a relatively thin sheath of flexible material around each filament so that the sheath provides positive contact between adjacent windings even during deformation of the winding caused by rotation of the structure.
Hatch, U.S. Pat. No. 4,080,845 discloses a composite flywheel with a conical or concave shape. Upon rotation, the surfaces of the flywheel tend to flatten with a resultant increase in radius measured from the axis of rotation. Such an increase in radius, while permitting the disc to flatten, is said to have the effect of substantially eliminating radial stresses within the flywheel. Such a flywheel is stated to be readily prepared from pre-impregnated fiber bundles or tapes.
Chevrolat et al, U.S. Pat. No. 4,138,286, teaches the preparation of a flywheel by winding a continuous filament impregnated with a hardenable polymer onto a mandrel. By the use of such a filament, it is suggested that polymerization will have progressed sufficiently at the end of the winding operation so that the polymer will not flow during the hardening operation. In addition it is suggested to use windings of at least two filaments having different moduli of elasticity so that the filament which has the lowest modulus of elasticity is in the portion nearest the mandrel and the filament which has the highest modulus of elasticity is in the peripheral portion. In addition it is suggested that the winding of a filament can be carried out under a constant or slightly decreasing tension from the central zone to the periphery.
Knight, Jr. et al, U.S. Pat. No. 4,187,738 is directed to an improved rim for a flywheel. The rim is fabricated from resin impregnated filamentary material which is circumferentially wound in a side-by-side relationship to form a plurality of discretely and sequentially formed concentric layers of filamentary material that are bound together in a resin matrix. Each layer is prestressed to a prescribed tension loading during winding and then cured prior to forming the next layer such that when finished, the various layers will have been placed in a distribution which counterbalances the radial tensile stresses generated during rotation of the rim.
From the above, it can be seen that various more or less complicated procedures have been proposed for dealing with the problem of delamination. The attempted solutions discussed above fall in three general categories.
In the first category, the rings of the flywheel are bound at a small number of locations which allows the flywheel to change shape during rotation, or in the alternative, the flywheel is wound using a schedule of increasing prestress levels in concentric layers. However, since such flywheels must be made from fibers with multiple winding being required for each level, the number of bindings which must be made or the need to cure between each layer results in a large increase in the production time of a flywheel.
The second category is based upon a relaxation of the assumption that the modulus of the elements must be constant throughout the flywheel. These attempted solutions either explicitly state that the filament having the lowest modulus should be closest to the axis, or in the alternative, suggest that their structures overcome such a need for varying the modulus due to the structure disclosed.
The third category is that of a ballasted flywheel. In such an approach, the stress is made more uniform by adding ballast near the center of the flywheel, usually in the form of heavy powders such as lead, with a slight penalty in the energy/weight storage. However, adding ballast to the flywheels of the prior art is rather difficult due to the use of fine filaments such as Kevlar (trademark of DuPont de Nemours and Company for an aromatic polyamide fiber) or graphite.
As discussed above, most of the prior art discusses the use of filaments such as Kevlar (trademark of DuPont de Nemours and Company for an aromatic polyamide fiber) and graphite. However, certain of the above, including Hatch and Rabenhorst U.S. Pat. No. 3,964,341 mention the use of "tapes". However such tapes are in actuality flat prepregs, i.e., filamentary elements within a matrix formed into a flat tape. Such tapes must be partially cured before being wound into a flywheel and are not homogeneous across the tape width. Rather, the strength elements are not continuous at any point across the width, being distributed instead on a discontinuous basis.
Accordingly, a need exists for composite flywheels which are more readily manufactured while also having a reduced susceptiblity to delamination.